1. Technical Field
The invention includes embodiments that relate to a propulsion system. The invention includes embodiments that relate to method of using the propulsion system.
2. Discussion of Art
Some vehicles use electric traction motors to propel the vehicle. These electric traction motors may be connected to an electrical coupling structure referred to as a link that provides the motors with power. Such links may be an electrical bus system. One or more electrical generators on-board the vehicle may be used to provide the power to the link. In certain operating conditions, such as when the vehicle is decelerating or is maintaining speed on a downhill grade, the back-electromotive force (EMF) produced by the electric motors is greater than the voltage provided by the engine-driven alternator. Under such conditions, the electric traction motors cease acting as motors and become generators. This process, known as dynamic braking, is a form of electric braking that is used to reduce wear on the mechanical brake system components of a vehicle. In the case where the vehicle is a locomotive, dynamic braking reduces brake wear on the locomotive and also all of the rail cars of the train. During dynamic braking, one or more resistors dissipate the electric power as heat. In the case where the vehicle is a truck or other medium or heavy duty vehicle using electric drive propulsion, dynamic braking can be used to reduce wear on the mechanical brake system components associated with each individual wheel.
Hybrid propulsion systems have been developed to recover some of the energy that would otherwise be wasted as heat during dynamic braking. The recovery of this otherwise-wasted energy is regenerative braking. Hybrid propulsion systems can use two different energy sources: a heat engine and an energy storage unit. The heat engine may burn fuel to produce mechanical work—an internal combustion engine, a turbine engine, and a diesel engine are examples. The energy storage unit may include an electrically re-chargeable battery, an ultracapacitor, or a flywheel having a high power density.
Conventionally, a hybrid propulsion system includes an on-board heat engine coupled to an alternator that converts the mechanical output power of the engine into an alternating current (AC) electrical power. A rectifier converts the AC output of the alternator into a direct current (DC). A portion of the DC output of the rectifier may charge an energy storage unit, and a remaining DC output portion drives an electrical motor, which in turn propels the vehicle.
During vehicle acceleration, the energy storage unit may discharge to augment electrical power output of the engine-alternator, and may provide boosted power levels for a period of time. During vehicle braking, the energy storage unit re-charges to regeneratively capture a portion of the energy otherwise-wasted during braking.
Heavy duty hybrid propulsion systems may require massive energy storage devices. Due to the simultaneous power and energy requirements, conventional heavy duty hybrid propulsion system may need parallel operation of many smaller energy storage units that are normally configured as several parallel strings of a large number of series-connected cells. Power sharing within the parallel energy storage units may be problematic.
It may be desirable to have a propulsion system with properties and characteristics that differ from those properties and characteristics of currently available propulsion systems. It may be desirable to have a propulsion system that implements a method that differs from those methods currently available.